1. Technical Field
This invention relates to gas turbine engine rotor assemblies in general, and to apparatus for inhibiting frictional wear between mating titanium alloy substrates such as a rotor blade root and rotor disk slot, in particular.
2. Background Information
A conventional rotor stage of a gas turbine engine includes a disk and a plurality of rotor blades. The disk includes an inner hub, an outer hub and a web extending between the two hubs. The outer hub includes a plurality of blade attachment slots uniformly spaced around the circumference of the outer hub. Each rotor blade includes an airfoil and a blade root. The blade root of each blade is received within one of the blade attachment slots disposed within the disk. A variety of attachment slot/blade root mating pair geometries (e.g., dovetail, fir-tree) can be used.
Gas turbine rotor stages rotate at high velocities through high temperature gas traveling axially through the engine. The high temperature, high velocity environment places a great deal of stress on each blade root/attachment slot pair. For example, centrifugal force acting on each blade will cause the blade root to travel radially within the attachment slot as a load is applied and removed. In a similar manner, vibratory loadings can cause relative movement between blade root and attachment slot. In both cases, the relative motion between blade root and attachment slot is resisted by the mating geometry and by friction. The friction, in turn, causes undesirable frictional wear unless appropriate measures are taken.
The undesirable frictional wear referred to above predominantly consists of a "galling" process and/or a "fretting" process. Metals used in the manufacture of gas turbine rotor assemblies such as titanium, nickel, and others form a surface oxide layer almost immediately upon exposure to air. The oxide layer inhibits bonding between like or similar metals that are otherwise inclined to bond when placed in contact with one another. Galling occurs when two pieces of metal, for example a titanium alloy blade root and a titanium alloy blade attachment slot, frictionally contact one another and locally disrupt the surface oxide layer. In the brief moment between the disruption of the surface oxide layer and the formation of a new surface oxide layer on the exposed substrate, metal from one substrate can transfer to the other substrate and be welded thereto. The surface topography consequently changes further aggravating the undesirable frictional wear. Fretting occurs when the frictional contact between the two substrates disrupts the surface oxide layer and the exposed metal begins to corrode rather than exchange metal as is the case with galling.
In some applications, galling can be substantially avoided by positioning a dissimilar, softer metal between the two wear surfaces. The softer metal, and oxides formed thereon, provide a lubricious member between the two wear surfaces. Simply inserting a softer metal between the wear surfaces does not, however, provide a solution for every application. On the contrary, the lubricious member must be tolerant of the application environment. In the high temperature, high load environment of a gas turbine engine rotor, the choice of a lubricious medium is of paramount importance. The lubricious member must: 1) minimize galling and fretting between titanium and titanium alloys substrates; 2) tolerate high temperatures; and 3) accommodate high loads.
U.S. Pat. No. 4,196,237 issued to Patel et al. (hereinafter referred to as Patel) reports that a disadvantage of an aluminum bronze (Al-Bronze) coating as an anti-gallant is that such a coating has a relatively low hardness. Patel further reports that a spray powder alloy which includes minor percentages of Ni, Fe, Al, and a majority percentage of Cu avoids the complained of hardness problem. In fact, Patel reports test results which include an evaluation of a 88% Cu--10% Al--2% Fe alloy sprayed onto a 1020 steel substrate (a metal not well suited for gas turbine rotor applications), as well as other similar alloys which include up to 10% Ni sprayed on the same steel substrate. Patel indicates that the sprayed alloys containing Ni showed a "marked improvement" in hardness and wear resistance relative to the alloy without the Ni when applied to a 1020 steel substrate.
U.S. Pat. No. 4,215,181 issued to Betts (hereinafter referred to as Betts) discloses a method for inhibiting the effects of fretting fatigue in a pair of opposed titanium alloy mating surfaces. Betts indicates that copper shims provide beneficial protection from fretting when placed between the two opposed titanium alloy mating surfaces. Betts further indicates that a shim comprising an Al-Si-Bronze alloy did not prevent fretting fatigue of the substrates. In fact, Betts reports that the fatigue life of the specimen was essentially the same as that for the bare titanium fretting fatigue. A disadvantage of using a shim is that the shim, or a portion thereof, can dislodge and cause the then unprotected wear surfaces to contact one another. In a gas turbine engine application, a dislodged shim (or portion thereof) can cause undesirable foreign object damage downstream.
Al-Bronze alloy anti-gallant coatings have been applied to nickel alloy stator vane rails and feet to prevent galling between the stator vanes and iron alloy outer casings. The load stresses in the stator vane applications are of a different nature than those between a rotor blade root and a rotor disk slot. Specifically, the centrifugal loading on the rotor blade creates a much higher load, and are much more localized, than that between the stator vane and the outer casing. The rotor blade is also subject to a high cycle motion, and consequent high cycle friction.
What is needed, therefore, is a method and apparatus for inhibiting the effects of frictional wear in a rotor blade root/attachment slot pair, one capable of performing in a gas turbine engine environment, one that can be used with titanium alloy substrates, one that minimizes the opportunity for foreign object damage with in a gas turbine engine, and one that is cost-effective.